The present invention relates to a control device for an internal-combustion engine mounted on an automobile or the like, and more particularly to a control device intended to reduce, in an in-cylinder injection internal-combustion engine for directly injecting fuel into a combustion chamber, torque down caused upon resuming supply of fuel in fuel cut mode.
In recent years, in order to improve fuel consumption by improving the fuel consumption rate, an internal-combustion engine (engine) capable of being operated at a leaner air-fuel ratio than the theoretical air-fuel ratio, i.e., lean air-fuel ratio has been developed and put into practical use.
Accordingly, in an engine operable at a lean air-fuel ratio, an air-fuel mixture within a combustion chamber is stratified by contriving the shapes of the combustion chamber and intake port and the fuel injection system, whereby an air-fuel mixture with high fuel concentration is gathered close to the ignition plug as possible to improve the ignitability. When it becomes thus possible to suitably stratify the air-fuel mixture, it becomes possible to make the entire air-fuel ratio lean by making only the fuel concentration of the air-fuel mixture near the ignition plug high, that is, to make it rich. Also, the air-fuel ratio can be freely controlled within a wide range.
On the other hand, in order to further improve the fuel consumption rate, a control (fuel cut mode) for stopping supply of fuel into the combustion chamber in an engine is effected when a decelerating state of a vehicle is detected from operating condition. In the fuel cut mode, since a sufficient feeling of deceleration cannot be obtained when there is a large amount of air, the amount of intake air is also reduced especially in case the engine is operated in a lean air-fuel ratio and the amount of intake air is increased and corrected to an amount of intake air required for the lean air-fuel ratio. When the vehicle decelerates and reduces the engine rotation speed to a predetermined rotation speed, supply of fuel is resumed for maintaining the engine idle operating condition.
In the fuel cut mode, which stops supply of fuel to the engine combustion chamber, torque down is prevented by somewhat increasing the fuel concentration of the air-fuel mixture when the engine rotation speed reduces to a predetermined rotation speed to resume supply of fuel. To increase the fuel concentration of the air-fuel mixture, however, there is a limit, and the torque down is not sufficiently prevented under the present conditions. Especially in an in-cylinder injection internal-combustion engine in which fuel injection is effected in the compression stroke, since too high air-fuel ratio may cause accidental fire, the fuel concentration of the air-fuel mixture cannot be made too high.
Also, to secure any amount of air on resuming supply of fuel, it is also conceived to suppress reduction in the amount of air in the fuel cut mode. When, however, the reduction in the amount of air is suppressed in the fuel cut mode, the pressure within the intake manifold becomes high, and there will be a large amount of air to cause defective deceleration (a feeling of free running).
Thus, as means for improving combustion stability on returning from the fuel cut mode, such one as described in, for example, Japanese Patent Laid-Open Application No. 4-325742 has been conventionally known. In the engine disclosed in the aforesaid official gazette, when the engine rotation number N exceeds a predetermined rotation speed N1 and an accelerator pedal switch is ON, the engine is determined to be in a decelerated state and the fuel cut is effected. When the engine rotation speed N becomes below the predetermined rotation speed N1 with the accelerator pedal switch in an ON state during the fuel cut, the throttle opening .theta. is made larger by a predetermined amount C to gradually open the throttle opening .theta.. Then, when the throttle opening .theta. exceeds a map value K for control during deceleration, fuel-supply is resumed at a stage where the throttle opening .theta. is fixed to the map value K, whereby accidental fire is prevented to secure the combustion stability.
When fuel-supply is effected after the opening of the throttle valve or the like is opened to a predetermined opening on returning from the fuel cut mode as described above, it becomes possible, for the time being, to prevent accidental fire and to secure the combustion stability. Under such control, however, merely the commencement of release of the throttle valve during fuel cut and start of fuel-supply is determined depending upon whether or not the engine rotation speed N exceeds the predetermined rotation speed N1, and the engine rotation speed during fuel-supply is not taken into consideration. Therefore, the engine rotation speed at which fuel is supplied on returning from the fuel cut does not become constant when a rate of change in the engine rotation speed is different. If the rate of change in the engine rotation speed is high, the throttle opening .theta. exceeds the map value K, and no fuel is supplied although the engine rotation speed N has greatly lowered to return to a specified amount of air, thus possibly causing torque down resulting in engine stall. Also, if the rate of change in the engine rotation speed N is low when a predetermined rotation speed N1 for determining the aforesaid fuel cut is set to be high in order to eliminate the aforesaid defect, there is a possibility that although it does not cause an engine stall but it is possible to cut the fuel, fuel-supply will be started to deteriorate the fuel consumption.
The present invention has been achieved in the light of the aforesaid conditions, and its object is to provide a control device for an internal-combustion engine capable of improving the fuel consumption while reducing torque down caused upon resuming supply of fuel in the fuel cut mode.